A. Field of the Invention
The invention relates to the art of relatively rotating face ring-type shaft seals and, more particularly, to seals for isolating high pressure abrasive-containing drill fluid, such as mud, for a fluid-driven downhole drill assembly from the bearing chamber for the rotary bit shaft.
B. Prior Art
Heretofor the wear life of bearings for fluid-driven downhole drill assemblies has been quite short because of the inability to seal these bearings from the abrasive drilling fluid such as mud. These drill assemblies include a drill string held stationary in a well to conduct a pressurized flow of motive drill fluid downwardly into the hole. Near the bottom of the drill string, there is a hydraulically operable motor or turbine which is motivated or propelled by the drill fluid. This motor or turbine in turn rotates a vertically disposed shaft, extending within the lower end of the drill string and driving the bit used to drill the well. Upon passage through the motor or turbine section, the drill fluid is passed out through the bit on the lower end of the shaft and flow upwardly through the well hole.
It is necessary to provide some form of bearing means in the annular space between the rotating shaft and the surrounding lower end of the stationary drill string. Unless these bearings are protected against the ingress of abrasive particles in the drilling fluid, which is typically in the form of mud, these bearings become permanently damaged. Since their replacement requires raising and lowering the drill string at great expense, many efforts have been made to exclude drilling fluid from the bearings by isolating them with seal means at opposed ends of the annular bearing chamber. Since the bearings often require lubricant, the seal means must also contain the lubricant in the bearing chamber.
An effective and economical seal means to protect the bearing chamber for drill bit shafts is a fundamental problem in the drilling art. If packing seals are provided at the top and bottom of the bearing chamber to confine the lubricant and prevent ingress of abrasive particles, these packing seals in turn are subject to the abrasive particles carried by the drill fluid and, consequently, quickly become worn to such an extent as to leak and allow the abrasive particles to enter the bearing chamber. Entry of the abrasive particles into the bearings ground up the bearings even when these were made of hard materials such as tungsten carbide. It has even been heretofore proposed that the bearing chamber lubricant be pressurized in accordance with the ambient drilling fluid pressure by means of a freely movable partition wall between the bearing chamber and drilling fluid to prevent drilling fluid leakage into the bearing chamber. This concept is illustrated in U.S. Pat. No. 4,019,591 to Fox. However, a disadvantage with this type of arrangement is that bearings must then operate in a lubricant environment of fluctuating pressures and generally very high drill fluid pressures. A further disadvantage regarding pressurization of the bearing chamber is that there is an opportunity for lubricant leakage from the bearing chamber due to the pressure differential across the bearing chamber caused by the drop in drill fluid pressure between the turbine or motor section side of the bearing chamber and that bearing chamber side facing the well hole space surrounding the drill bit.
It would therefore be a great improvement in the art to provide means which would effectively seal the bearings and bearing lubricant chambers for the shafts of fluid-driven drills so as to keep the bearings free from the abrasive particles in drilling fluid and contain the supply of lubricant for the bearings.